The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Natural resources such as oil and gas residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is employed to plug and seal off undesirable flow passages in the cement sheath and/or the casing.
One problem commonly encountered during the placement of a cement slurry in a wellbore is unwanted gas migration from the subterranean formation into and through the cement slurry. Gas migration is caused by the behavior of the cement slurry during a transition phase in which the cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. When first placed in the annulus, the cement slurry acts as a true liquid and thus transmits hydrostatic pressure. However, during the transition phase, certain events occur that cause the cement slurry to lose its ability to transmit hydrostatic pressure. One of those events is the loss of fluid from the slurry to the subterranean zone. Another event is the development of static gel strength, i.e., stiffness, in the slurry. As a result, the pressure exerted on the formation by the cement slurry falls below the pressure of the gas in the formation such that the gas begins to migrate into and through the cement slurry. Eventually the gel strength of the cement slurry increases to a value sufficient to resist the pressure exerted by the gas in the formation against the slurry.
The flow channels formed in the cement during such gas migration undesirably remain in the cement once it has set. Those flow channels can permit further migration of fluid through the cement. Thus, the cement residing in the annulus may be ineffective at maintaining the isolation of the subterranean formation. As such, gas may undesirably leak to the surface or to other subterranean formations. An expensive remedial squeeze cementing operation may be required to prevent such leakage. However, the gas leakage may further cause high volume blow-outs shortly after cement replacement and before the cement has initially set.
In an effort to suppress gas migration, cement slurries have been designed that include metal particles such as an aluminum powder for generating a stabilized, dispersed gas. The gas is often generated in situ in a cement slurry by reacting the metal particles with an alkaline solution, e.g., the cement slurry, and/or water to yield hydrogen. A sufficient amount of gas is formed in the cement slurry to prevent the migration of gas into or through the slurry before it has sufficiently gelled to resist such migration.
The metal particles contained in the cement slurry are usually encapsulated with an inhibitor for delaying the hydrogen-generating reaction until a desired time such as after the slurry has been placed in its desired location in the wellbore, e.g., the annulus. Ideally, the inhibitor effectively inhibits the particles from interacting and reacting with oxygen, water vapor, and the cement slurry until gas generation is desired. Examples of chemical reaction inhibitors commonly used to encapsulate the reactant metal particles, particularly aluminum powder, are fatty acids of sorbitan, glycerol, and/or pentaerythritol such as sorbitan monooleate. Additional information relating to the use of metal particles to generate gas in cement slurries and/or inhibitors to retard the generation of the gas may be found in U.S. Pat. Nos. 5,718,292, 4,565,578, 4,450,010, 4,367,093, and 4,340,427, and in U.S. Patent Application Publication No. 2004/0221990 A1, each of which is incorporated herein by reference.
Unfortunately, metal particles coated with such inhibitors suffer from the drawback of undergoing severe sintering when they are not flowable such as when they are being stored. As used herein, “sintering” refers to the agglomeration of metal powders at temperatures below the melting point. Such sintering may be facilitated by the non-uniformity of the inhibitor coating, mechanical vibration of the particles such as when they are being transported, the compaction of the particles in a container, and/or the exposure of the particles to relatively high temperatures, air, oxygen, and/or moisture. As a result of such sintering, the metal particles are neither free flowing as before nor properly encapsulated with the inhibitor, making the particles extremely reactive. They may react with water vapor and release the hydrogen prematurely, or they may bond with oxygen to form metal oxides, precluding them from later forming hydrogen gas. The duration for which the particles can be stored without undergoing any changes in their physical (e.g., free flowing nature) or chemical properties, which is known as the shelf life of the particles, thus is often shorter than desired. A need therefore exists to develop an improved way of delaying the reaction of the metal particles and thereby improve the shelf life of such particles.